1. Field of the Invention
Embodiments of the present invention relate generally to oscillators and methods of adjusting the oscillation frequency of the oscillators. More particularly, embodiments of the invention relate to oscillators and methods of adjusting the oscillation frequency of the oscillators to compensate for changing external environmental factors such as temperature, and power supply voltage levels.
A claim of priority is made to Korean Patent Application No. 2005-60836 filed on Jul. 6, 2005, the disclosure of which is hereby incorporated by reference in its entirety.
2. Description of Related Art
An oscillator is a device used to generate a continuous output waveform. For example, an oscillator typically comprises a tuned electronic circuit adapted to generate an alternating current such as a periodic signal. Most oscillators can be roughly classified into one of two groups: external oscillators such as crystal oscillators, and internal oscillators included in semiconductor devices.
Historically, oscillators have been used in a variety of electronic devices such as computer systems, semiconductor devices, and communication devices. More recently, oscillators have been adopted in a number of in portable electronic devices such as cellular phones, personal digital assistants (PDAs), smart batteries of cellular phones, and so on. In order to meet the performance requirements of such portable devices, much research has been devoted to designing oscillators having lower power consumption, lower cost, and improved output accuracy and stability.
FIG. 1 is a circuit diagram illustrating a typical configuration of a conventional oscillator.
Referring to FIG. 1, an oscillator 10 includes a reference voltage generator 1, a reference current generator 2 and an oscillation unit 3. Oscillation unit 3 includes a comparator 4, a capacitor 5 and a switch 6.
Reference voltage generator 1 generates a reference voltage “Vr”, reference current generator 2 generates a reference current “Ir”, and oscillation unit 3 generates an oscillation signal “Osc” with a logic state based on a comparison between reference voltage “Vr” and a comparison voltage “Vc” produced by reference current “Ir”.
Comparator 4 has a reference input terminal coupled to an output terminal of reference voltage generator 1 and a comparison input terminal coupled to an output terminal of reference current generator 2. Capacitor 5 and switch 6 are coupled in parallel to the comparison input terminal of comparator 4.
When capacitor 5 is charged by reference current Ir, comparison voltage “Vc,” which is apparent at the comparison input terminal of comparator 4, increases. When comparison voltage Vc is higher than reference voltage Vr, switch 6 is turned on in response to a change in the logic state of oscillation signal Osc, and comparison voltage Vc rapidly decreases to a ground voltage by discharging through switch 6.
Switch 6 is turned off in response to a change in the logic state of oscillation signal Osc after comparison voltage Vc is discharged through switch 6. The above sequence of voltage changes is repeated so that oscillation signal Osc continues to oscillate between a first logic state and a second logic state.
The oscillator typically provides a timing signal function (e.g, a clock) for an electrical system. Accordingly, if the output frequency of the oscillator does not correspond to a target frequency of the system, the system may not function correctly.
The output frequency of the oscillator can vary based on a number of factors such as the process conditions used when manufacturing the oscillator, and external environmental factors such as power voltage and temperature, to name but a few. For example, the output frequency of an oscillator may not be stable over a wide temperature range of about −30° C. through 140° C.
To avoid problems arising from instability, conventional oscillators often provide various compensation circuits. For example, a reference voltage generator sensitive to power voltage and temperature may be replaced by a high-cost product that is insensitive to temperature differences. Also, compensator circuits may be configured to compensate for temperature differences by generating a current that is in inversely proportional to a temperature of the reference current generator.
Unfortunately, these compensation circuits are generally complex, and they tend to increase the power consumption, test times, and cost of the oscillator.